In prior art vacuum cleaners equipped with such cyclonic dirt collectors as described in, e.g., Japanese Laid-Open Publication Nos. 2000-342492 and 1997-253011, dirt entraining air is separated into dirt and clean air by a cyclonic air flow in a cyclonic dirt collector and thus separated dirt then merely piles up at a lower portion of the cyclonic dirt collector.
Therefore, if a large amount of fibroid material, e.g., cotton or the like, is included in the dirt containing air, thus separated dirt tends to occupy a large volume relative to its mass. In a dirt separating process, dirt with a large mass rotates along an outer peripheral side of the cyclonic dirt collector due to the centrifugal force generated by the cyclonic air flow in the cyclonic dirt collector and, therefore, does not considerably reduce the amount of suction air flow which passes through a main filter installed at an inner peripheral side of the cyclonic dirt collector. However, fibrous type dirt having a relatively small mass may not be readily separated from the dirty air in the centrifugal separation process of the cyclonic dirt collector and consequently may be readily stuck on a surface of the main filter, thereby decreasing the amount of suction air flow. In other words, the suction efficiency of a vacuum cleaner can be rapidly deteriorated by the intake of air containing dirt having a relatively small mass.
Such a problem may be addressed by increasing the size of a dirt collector to accommodate a sufficient amount of dirt with a small mass; however, such would not be advantageous because increasing the volume of the dirt collector is not only cumbersome but also against the current trend for the smaller and lighter vacuum cleaners.